Helix antenna

The do it thyself defacto standard was established by the AO40, Inmarsat, and various WX satellites. All are right hand circular polarization for the ground station.

Be careful if you build a helical feed for a dish or other reflector antenna. The reflection changes the sense, so a dish would need a left hand circular polarized feed.

Right hand.

Please post how it works out. Also, have you considered building a 4

Yes, I thought about that but I have plans for the 40 turn unit including the matching device. Also I have access to a Network Analyzer and S pararamter test set for tuning it up. I think building the matching network may be easier than build a phasing harness for 4 Helix antennas. However I have never done this before so plans may change. Who knows, if things work out OK I may build a 4 X 40 turn device.

Jimmie

My guess is paralleling the 4 helix antennas would have a broader bandwidth than the impedance matched solution. Broadband is good in the sense that it allows for more error in the construction.

Usually paralelling(stacking) antennas means a narrower bandwidth. This statement assumes all the stacked antennas are identical.

Jimmie

This should be of interest to anyo one wanting to build a helix antena.

I see no reason for stacked antennas to have a narrower bandwidth. Can you elaborate?

When I build antennas, I try to use all copper if I can. Otherwise, you get a battery connecting Al to copper. I'm not sure about connecting copper to galvanized. I've built a few log periodics (VHF/ UHF) and used that gunk (OK, not a technical term) used with Al wiring. Still, it deteriorates with time. Consider using copper wire.

Have you considered the biquad? Unless you have circular polarization on both ends, you will lose 3db when interfacing with linear polarized antennas, so 16db becomes 13db, which is close to the biquad.

I guess a long helix would be better for snooping purposes since not everyone is vertically polarized.

If your definition of bandwidth is the frequency between the -3dB points, stacking two isolated resonant antennas together results in exactly the same bandwidth as one antenna. An easier way to see this is if the two antennas were simple parallel tuned circuits acting as a bandpass filter. If you connect them together, using some means of coupling that is critically coupled (i.e. maximum power tranfer), the resultant circuit has exactly the same bandwidth. If you plot it on a piece of graph paper, you could stack a dozen critically coupled tuned circuits together and get exactly the same -3dB bandwidth. Obviously the skirt factor and bandwidth at other refrence points will be narrower as you add sections.

Of course, such things fall apart when dealing with real world devices and antennas. Two stacked antennas will couple to each other, causing difficulties with such simplistic explanations. The traditional 2x2 array of helixes heavily couple to each other, especially since they're the same sense. Anyway, the only way to get it right is to fire up your favorite NEC antenna modeling program, which takes such things into consideration. 4NEC2 includes a helix generator.

Specifically for a helix, the approximate -3dB bandwidth for a single helix is roughly equal to the center frequency. In other words, if you cut a helix for 2.4GHz, it will be usable from 1.2 to 3.6Ghz. Stacking 4 of these together will theoretically not reduce this bandwidth, but in reality, will reduce it somewhat. I don't think operation in an 83.5MHz band is going to be affected with an antenna with a 1 or 2Ghz bandwidth.

How about the impedance matching scheme? That must narrow the bandwidth. Also your comments on mixing Al and Cu?

Sorry I was thinking of bandwidth in terms of frequency versus VSWR. This is usally the limiting factor as gain bandwidth is almost always much broader than VSWR bandwidth. As of yet I dont know whether this will be a serious consequence or not concerning the helix antenna.

My plan is to build the antenna and tune it for greatest field strength

A few years ago I stacked 2 2 meter 6 element quad antennas. I had plans to stack 4 but didnt do it because of the VSWR excursion from one end of the band to the other. My solidstate amp was not fond of the 2 to 1 VSWR near the band edges.

In this situation the VSWR bandwidth of 2 antennas was less than 1 antenna and with 4 antennas it was even less. I am sure the array would have still exhibited considerable gain if I had a way to readily correct for the impedance changes seen by my amplifier.

Jimmie

I wonder if the VSWR effects were due to coupling between antennas?

e quoted text -

quoted text -

I dont know, the antennas came with printouts from NEC antenna modeling software. One of the parameters given was a diagram of their capture area. The antennas were place according to this data with their capture areas just touching. A local ham who is very much into antenna modeling verified the data that came with the antennas. At the time I was surprised by the reccomenced spacing thinking it was excessive until I discovered that rules saying 1/2 or 5/8 spacing was not always correct.

ide quoted text -

de quoted text -

Source of info on stacking antennas

Jimmie

ty of a helical

Hide quoted text -

Hide quoted text -

I could see the stacked yagis interacting. The problem with beam antennas is from the top and bottom, they more or less look like dipoles because the driven element is not shielded from those directions. I never stacked any beams, but if I did, I'd consider antenna designs that had what amounts to a corner reflector at the back of each antenna. That would shield the driven element.

I've been trying to DF vertically polarized UHF signals with a yagi. I find it really hard to find a peak and suspect there is just too much gain from the dipole as view from top and bottom. I've DFed HF with loop using the null, and that works very well. The particular UHF signal is heavily modulated, so the cheap double ducky box doesn't work.

rity of a helical

.- Hide quoted text -

- Hide quoted text -

I use a "Little L-per" direction finder and sometimes use a Yagi with a little handheld VHF UHF radio. When I use the YAGI sometimes I turn it around backwards and search for the null instead of the peak. Also you need to check the pattern of your yagi. Some can be rather skewed from what you think they should be.

Jimmie

larity of a helical

know whether I

nect with other

ed building a 4

think building

harness for 4

on.- Hide quoted text -

s?- Hide quoted text -

I've tried the backward yagi. The trouble is you get a lot of reflection. One oddball scheme I have used with control channels, ie. digital signals, is to view the eye pattern via software demodulation. The widest open eye corresponds to the cleanest and thus most direct signal. I've also done the "remove the antenna" trick once you get close.

The L-per is a bit expensive for hobby use unless there are DIY scheme.

polarity of a helical

to know whether I

onnect with other

ered building a 4

I think building

ng harness for 4

tion.- Hide quoted text -

nas?- Hide quoted text -

A variable attenuator in line with the Yagi will help a lot with reflections.

I have one of those binary switch selected attenuators. I think it can switch in 3-20 db pads a 10, a 5, a 3 . If you can monitor the AGC voltage on the rx with a voltmeter this give a much better indication of the strongest signal than an S meter.

I work at an airport an people used to think I was nuts but when I used to look for Emergency Locator Transmitters I would start by climping the Control Tower. By getting up high I didnt have as much problems with reflections and could often point directly at the offending aircraft from the catwalk of the tower.

Jimmie

e polarity of a helical

d to know whether I

connect with other

idered building a 4

ets around the

. I think building

sing harness for 4

re so plans may

d a 4 X 40 turn

and is good in

uction.- Hide quoted text -

ennas?- Hide quoted text -

I have a HP RF attenuator. I forgot to bring it with me when I was DFing these repeater sites, but will do so this time. I used a Minicircuits 4 way splitter as an adhoc attenuator, but it wasn't enough.